1. Field of the Invention
The present invention relates generally to an apparatus and method for selecting antennas in a Multi-Input Multi-Output (MIMO) system, and in particular, to an apparatus and method for selecting antennas in an Orthogonalized Spatial Multiplexing (OSM) system.
2. Description of the Related Art
Generally, MIMO systems provide very important means to increase the spectral efficiency for wireless systems. Particularly, spatial multiplexing schemes enable extremely high spectral efficiencies by transmitting independent streams of data simultaneously through multiple transmit antennas. In order to fully exploit the potential of multiple antennas, a receiving entity can apply full Channel State Information (CSI) knowledge to a transmitting entity optimize a transmission scheme according to current channel conditions.
Based on knowledge of the full CSI at a transmitter, a basic idea of preceding has been proposed. Most work on these closed-loop MIMO systems is carried out by obtaining Singular Value Decomposition (SVD) of the channel transfer matrix.
It is well known that the optimum linear precoder and decoder decouple the MIMI channel into several independent eigen subchannels and allocate resources such as power and bits over theses subchannels.
More realistic assumptions about CSI at the transmitter and the receiver can impact the potential channel gain of MIMO systems. While CSI can be acquired at the transmitter by assuming channel reciprocity between uplink and downlink transmission in Time Division Duplex (TDD) systems, more often CSI needs to be obtained at the receiver and sent back to the transmitter over a reliable feedback channel.
However, in practical situations, the amount of feedback from the receiver to the transmitter should be kept as small as possible to minimize the overhead. In this sense, the assumption of full channel knowledge at the transmitter is not realistic, since even under flat-fading MIMO channels, the feedback requirements generally grow with the number of transmit antennas, receive antennas, and users. Another drawback of preceding systems is that the SVD operation requires high computational complexity and is known to be numerically sensitive.
To address these issues, the transmitter with limited feedback information in a communication system tries to utilize the system resources more efficiently. The transmitter precoder is chosen from a finite set of preceding matrices (hereinafter, called ‘codebook’), known to both the receiver and the transmitter. The receiver selects the optimal precoder from the codebook with a selection criterion based on the current CSI, and reports the index of this matrix to the transmitter over a limited feedback channel.
Recently, a new spatial multiplexing scheme, called Orthogonalized Spatial Multiplexing (OSM), for a closed-loop MIMO system, has been proposed, which allows a simple Maximum-Likelihood (ML) receiver. The interest is restricted to spatial multiplexing systems transmitting two independent data streams, which are important in practical wireless system designs. The ML Decoding (MLD) is optimal for detecting symbols in MIMO Spatial Multiplexing (SM) systems. However, its computational complexity exponentially increases with the number of transmit antennas and the size of constellations.